1. Field of the Invention
The present invention relates to a constant voltage generating circuit for generating a reference voltage needed by a circuit such as a power supply circuit, and in particular to a constant voltage generating circuit that can detect the starting of a low voltage without fail despite operating from a supply voltage lower than is conventionally required.
2. Description of the Prior Art
Circuits that operate with high accuracy and stability from a wide range of operating voltages and in a wide range of operating temperatures, such as power supply circuits, D/A converters, and A/D converters, require as their reference voltage a constant voltage that is highly accurate and stable. For example, in a PWM-type switching regulator, the accuracy and stability of the reference voltage fed thereto greatly affects, among others, the differential voltage amplification factor and the in-phase component suppression ratio of the error amplifier and the PWM comparator provided within the IC used in the switching regulator. For this reason, where high accuracy is required, the constant voltage generating circuit incorporated in the IC generates a constant voltage by the use of a band-gap circuit that exhibits almost no dependence on temperature.
Such a constant voltage generating circuit operates from a supply voltage input thereto, and accordingly the output voltage of the constant voltage generating circuit rises as the input supply voltage rises. Thus, whether the output voltage remains stable at a predetermined voltage or not is greatly affected by how the input supply voltage rises and how it fluctuates. Therefore, it is essential to check first whether the voltage generated by the constant voltage generating circuit has certainly reached the predetermined constant voltage and whether the input supply voltage has reached a voltage that permits the generation of the predetermined constant voltage, before feeding the constant voltage to circuits that need it as a reference voltage. Failing to do this may lead to malfunctioning and, with a power supply circuit, even destruction of the circuit connected as a load thereto or of the power supply circuit itself.
FIG. 3 shows an example of a conventional constant voltage generating circuit. In FIG. 3, the constant voltage generating circuit 10 generates an output voltage Vref by the use of, for example, a band-gap circuit 1. The output voltage Vref is fed to the inverting input terminal (xe2x88x92) of a comparator A1, and a division voltage Vd obtained by dividing an input supply voltage VIN with resistors R4 and R5 is fed back to the non-inverting input terminal (+) of the comparator A1. How the band-gap circuit 1 is configured and how it operates to generate the output voltage will be described later.
The output terminal of the comparator A1 is connected to the base of an NPN-type, common-emitter connection transistor Tr1 whose collector is pulled up through a resistor R3 to the input supply voltage VIN. When the output voltage Vref has reached a predetermined voltage, the transistor Tr1 outputs, at its collector, a low-level starting signal.
Next, with reference to FIG. 4A, how the conventional constant voltage generating circuit 10 operates will be described. FIG. 4A is a graph showing how the output voltage Vref and the division voltage Vd, obtained by dividing the input supply voltage VIN, vary according to the input supply voltage VIN. Along the horizontal axis is taken the input supply voltage VIN, and along the vertical axis are taken the division voltage Vd and the output voltage Vref. FIG. 4B shows the timing with which the conventional constant voltage generating circuit 10 produces a starting signal. Now, suppose that electric power starts being supplied. Then, as shown in FIG. 4A, in a region A, as the input supply voltage VIN rises, the division voltage Vd rises linearly in proportion thereto. The output voltage Vref also rises as shown in FIG. 4A, and reaches a stable voltage starting point T2, at which the output voltage Vref stabilizes, at the boundary between regions B and C.
However, the stable voltage starting point T2 varies greatly with variations in temperature and in the constants of the circuit elements. Thus, in the conventional circuit configuration, a starting signal is output after checking whether or not the input supply voltage VIN is sufficiently high to permit the output voltage Vref to certainly reach the stable voltage starting point T2 even under the influence of variations in temperature and in the constants of the circuit elements. Accordingly, in the comparator A1 shown in FIG. 3, the division voltage Vd of the input supply voltage VIN is compared with the output voltage Vref. Then, as indicated by a comparator detection point T3 in FIG. 4A, the comparator A1 outputs a high-level signal as late as when Vdxe2x89xa7Vref, i.e., when the region C has been passed through. This high-level signal turns the transistor Tr1 on, which then outputs, for example, a low-level starting signal as shown in FIG. 4B. The relationship Vdxe2x89xa7Vref holds also in the region A in FIG. 4A, but, in this region, since the input supply voltage VIN is very low, the comparator A1 does not operate and thus does not output a signal, and accordingly the transistor Tr1 is not turned on.
Likewise, when, as a result of electric power being shut off, or a fault in the circuitry, or a variation in the load, the input supply voltage VIN falls below the comparator detection point T3, the starting signal is stopped (turned to a high level) before it becomes impossible to keep the output voltage Vref at the predetermined voltage. In this way, it is possible to stop the operation of other circuits that use the output voltage Vref as a reference voltage before they start malfunctioning.
As described above, the stable voltage starting point T2, at which the output voltage Vref stabilizes, varies greatly with variations in temperature and in the constants of the circuit elements, and accordingly, in the conventional circuit configuration, a starting signal is output as late as at the comparator detection point T3 after checking whether or not the input supply voltage VIN is sufficiently high to permit the output voltage Vref to certainly reach the stable voltage starting point T2 even under the influence of variations in temperature and in the constants of the circuit elements. This makes it necessary to secure a margin, i.e., the region C shown in FIG. 4A. As a result, an IC circuit that uses as a reference voltage the output from a conventional constant voltage generating circuit cannot be designed to operate from a low input supply voltage, that is, it is impossible to lower the minimum operating voltage from which it can operate. This often makes it impossible to realize operation at a low voltage as required in information technology equipment.
An object of the present invention is to provide a constant voltage generating circuit that does not require the checking, as required in a conventional one, of whether or not the input supply voltage is sufficiently high to permit the generation of an output voltage stable at a predetermined voltage and that requires less circuit elements, operates from a lower input supply voltage, and thus consumes less electric power than a conventional one.
To achieve the above object, according to one aspect of the present invention, a constant voltage generating circuit is provided with: a reference voltage generating circuit of which that output voltage is so controlled as to be a constant voltage when the output voltage has risen, with an increase in the input supply voltage, to reach a predetermined voltage and that outputs the constant voltage as a reference voltage; a first transistor that turns on when the output voltage reaches the predetermined voltage in order to control the constant voltage output from the reference voltage generating circuit; a second transistor that is so connected that, when the first transistor turns on, a current proportional to the current flowing through the first transistor flows-through the second transistor; and a signal output circuit that detects the current flowing through the second transistor to output a detection signal indicating that the constant voltage is being output.
The second transistor may be given identical characteristics with the first transistor. Alternatively, the second transistor may constitute a current mirror circuit together with the first transistor.
The reference voltage generating circuit may be configured as a band-gap circuit that generates a band-gap voltage.
In a case where the reference voltage generating circuit is a band-gap circuit, the band-gap voltage output from the band-gap circuit is so controlled as to be a constant voltage as a result of a predetermined current flowing through the band-gap circuit. The first transistor controls the current flowing through the band-gap circuit so that the current is kept at a predetermined level by negatively feeding back the current when the output voltage reaches the predetermined voltage. The first and second transistors have bases thereof connected together and have emitters thereof connected together so that a current proportional to the current flowing through the first transistor flows through the second transistor. The signal output circuit outputs, as the detection signal, a signal based on the current flowing through the second transistor.
According to another aspect of the present invention, a constant voltage generating circuit is provided with: a reference voltage generating circuit that generates a reference voltage; an amplification control circuit that receives the reference voltage as an input voltage and amplifies the reference voltage so that an output voltage remains a constant voltage after having reached a predetermined voltage; a first transistor that turns on and permits a current to flow therethrough when the output voltage reaches the predetermined voltage in order to keep the output voltage constant; a second transistor that is so connected that, when the first transistor turns on and permits a current to flow therethrough, a current proportional to a current flowing through the first transistor flows through the second transistor; and a signal output circuit that detects the current flowing through the second transistor to output a detection signal indicating that the constant voltage is being output.
Here, the second transistor may be given identical characteristics with the first transistor. Alternatively, the second transistor may constitute a current mirror circuit together with the first transistor.
The reference voltage generating circuit may be configured as a band-gap circuit that generates a band-gap voltage.
The amplification control circuit may include an amplifier that controls the output voltage so that the output voltage is kept at the predetermined voltage by negatively feeding back a voltage commensurate with the output voltage and comparing the voltage so fed back with the reference voltage. In this case, the first transistor receives the signal output from the amplifier, and permits a current to flow therethrough when the output voltage reaches the predetermined voltage. The first and second transistors have bases thereof connected together and have emitters thereof connected together so that a current proportional to the current flowing through the first transistor flows through the second transistor. The signal output circuit outputs, as the detection signal, a signal based on the current flowing through the second transistor.